Bootcamp: Can't get into Mac OS

I've been running BootCamp on my MacBook Pro perfectly for months now but today I just can't seem to get into the Mac partition. I was playing a game on my Windows (Vista) partition for several hours then restarted the computer. As the start up drive is set to Mac, it started to attempt to load it. It was just stuck at the grey screen with the Apple in the middle and the endless rotating circle. Then as I waited longer the Apple turned into a "no access" symbol (the circle with a line across it) and the rotating circle just kept going and going. I waited several minutes but no luck.

So I held down the power button and this time instead of letting it auto boot Mac I held down Option and chose Mac drive myself (therefore the computer does recognize that the Mac partition is there). Same result. I rebooted into Windows (which it is still able to do with no problems at all) then went to the BootCamp assistant and pressed "Restart into Mac OS X". Same result.

I've googled it and no one seems to have had this problem. Any insight at all would be great. Thanks.

Was the OSX partition fine last time you were in it? Sounds like something went south somewhere maybe in the OSX partition. What OSX are you running? Do you have the Install DVD? If so, put in the OSX DVD and hold down C and boot into the installer and from the Utilities menu up top run Disk Utility and repair permissions and also check for problems.

Yes, it was running perfectly fine on Mac merely 3 hours prior to this. Unfortunately I don't have my CD with me as I'm away from home... but when I get my hands on it I will try what you said. Thanks for the response.

Oh, and I'm running Snow Leopard 10.6.8